Waterflood oil production is a secondary method of oil production which employs injection wells and production wells spaced in such a manner that water, or other suitable fluid, can be injected into the injection wells, and pressure therefrom will drive oil to the production wells. Eventually, waterflooding progresses to a point where the efficiency of oil recovery no longer is advantageous. Nevertheless, the oil-bearing formation contains in its pores a substantial amount of oil at residual oil saturation concentration. A portion of this residual oil can be displaced by microorganisms in a tertiary oil recovery method known as microbial enhanced oil recovery (MEOR). Such use of microorganisms for recovery of oil is disclosed, for example, by Zobell, U.S. Pat. No. 2,641,566, which teaches treating an oil-containing formation with hydrogenase-producing bacteria and recovering displaced oil by waterflooding. Using microorganisms for this purpose, said organisms are introduced with nutrients into the production wells and the wells are shut in for a period to allow metabolism to occur. It is known that the products of the microbes, metabolism cause some of the residual oil in the area of the microorganisms to be displaced from the pores of the oil-containing formation. This displaced oil can be recovered from the production wells by resuming the waterflood.
McInerney, U.S. Pat. No. 4,522,261, discloses the use of a strain of Bacillus licheniformis for MEOR. Bacillus licheniformis produces a biosurfactant which is useful for mobilizing oil. The surfactant also is disclosed to be useful when directly injected into the reservoir in a partially purified form from spent B. licheniformis nutrient media. This direct injection of the microbial product eliminates the need to inject microbes in order to increase tertiary oil recovery.
Plugging of the reservoir by microorganisms is a recognized problem. The prior art methods do not provide a simple and inexpensive method for injecting microorganisms in an injection well, such that reservoir plugging is avoided. Rather, microbes are commonly injected into production wells so that the probability of plugging the reservoir is remote; or they may be injected into an injection well, in the presence of an agent intended to prevent microbial-induced plugging of the reservoir. Pursuant to the latter method, Clark, U.S. Pat. No. 4,610,302 discloses injection of a sacrificial agent near the injection well in order to occupy the adsorptive binding sites of the reservoir rock which otherwise would bind microorganisms and cause severe plugging of the reservoir near the injection site. The sacrificial agent allows more of the reservoir matrix to be covered by injected microbes with minimal plugging. Alternatively, as noted, microbes can be injected at the site of the production wells; but this option inherently limits the volume of the formation that can be treated and, so, the amount of oil that can be mobilized.
Another related problem is that of ensuring survivability of microbes in the oil-bearing formation. First, certain microbes simply may not grow well in an oil-bearing rock formation. Second, even if microbes are potentially able to grow there, often, indigenous species can outcompete the injected species for nutrients, and thus inhibit survival of the injected species. Moreover, the problem of survivability is complicated when the use of more than one species of microorganism is desirable, in that the distinct species must be compatible with each other to ensure survivability of each species.
It would be desirable to provide an inexpensive method for microbial enhanced oil recovery using a combination of species of microbes that can be injected directly into an injection well without a sacrificial agent, such that the microbes mobilize oil, but do not plug the pores of the reservoir rock. Further, it would be desirable to provide a method for deriving a microbial formulation that survives well in an oil-bearing reservoir; efficiently displaces oil present at residual-oil-saturation concentration, without plugging the reservoir; and is not adversely affected by microbial species indigenous to the reservoir.